Biodegradable organic compounds in Yodo River water, the coagulated water and those ozonated waters were characterized on the concentration of assimilable organic carbon (AOC), the apparent molecular size and the sensitivity to E260 (UV absorbance at the wavelength of 260 nm). High molecular (>5,000 Da) organic compounds which were insensitive to E260 were contained with relatively high concentration in Yodo River, whereas they were not detectable as AOC. The AOC concentration in Yodo River was reduced by coagulation because the phosphorus concentration sharply decreased. The AOC concentration rapidly increased with the reduction of E260 and slowly with the depolymerization of high molecular organic compounds during the ozonation. However, 40-50% of AOC increasing with the ozonation was reduced by the coagulation. Furthermore, batch incubations of native bacteria living in the Yodo River were conducted in order to examine the validity of the apparent AOC concentration as an indicator of the regrowth potential. The AOC concentration calculated as acetate carbon equivalent would not exactly represent the concentration of the available carbon for the assimilation. However, maximum cell numbers for the incubation period were generally proportional to apparent AOC concentrations even if phosphorus limitation. In conclusion, it is suggested that coagulation is an effective process in controlling the concentration of AOC and is a useful indicator to evaluate the regrowth potential of heterotrophic bacteria in Yodo River.

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